Quick dehumidification of humid air or other process gases is an important issue associated with daily life and with numerous industrial applications.
Due to the increased public demands for the quality of air, food and medicines humidity control in the production and living environments has become increasingly important. In the industrial field, the existence of humid air and condensate may directly result in corrosion and malfunction of instruments and parts, or even failure of the corresponding process system. Meanwhile, humidity will inevitably lead to a change in the features of process materials with adverse effects on production. In our daily life, humidity is a fundamental factor in the growth of mold and a main factor for lesion. As a result, the transmission of pathogenic bacteria and pollutants to the air and the human body will be quickened. Studies show that the appropriate humidity for the human body (depending on the location and the seasonal ambient temperature) should be controlled to be 40-65%. The independent control of air humidity has become an inevitable development trend, however, in view of the energy crisis and environmental deterioration, it is obvious that development of a dehumidification process and system with high efficiency and energy conservation is the ultimate goal for dehumidification techniques.
The existing mature dehumidification techniques mainly include cooling dehumidification, liquid absorption dehumidification, solid absorption dehumidification, HVAC (heating, ventilation and air conditioning) dehumidification and the reel absorption dehumidification, developed on the basis of the solid absorption dehumidification techniques. With the transition of semiconductor cooling and heating techniques from the aeronautical and aerospace field to the civil field, the application of a novel dehumidification technique, thermoelectric cooling dehumidification has become more and more widely used to meet the demand for energy conservation and emission reduction.
Cooling dehumidification is to make use of a natural or artificial cooling source to cool humid air till reaching a temperature below the dew point, so as to remove the water vapor that exceeds the saturated humidity from the humid air in the form of condensate. The freezing dehumidifier is the most representative cooling dehumidification equipment. The freezing dehumidifier normally comprises of refrigerant compressor, evaporator, condenser, expansion valve, draught fan and air valve. This is the earliest and matures dehumidification technique featuring low initial investment, high COP, reliability, convenience and no need for a heat source. And this technique is the most widely used one in our daily life. However, as this technique adopts the Carnot cycle, the refrigeration agent eventually has an adverse impact on the environment. Also, despite the high COP, coupling process systems comprising multiple machines inevitably result in excessive consumption of electrical energy. This type of dehumidification system is also inappropriate for application at adverse ambient temperature (extremely high or low), and is not easy to maintain. Due to these problems of environmental impact and extremely high energy consumption, its application will be subject to more and more stringent control.
Liquid absorption dehumidification is to use a liquid drying agent to absorb vapor from humid air under the action of pressure gradient in view of the fact that the partial pressure of the vapor on the surface of the drying agent is lower than that in the humid air. This ensures consistent vapor partial pressure between the air and the agent. Liquid drying agent shall be dewatered for reuse. A typical liquid absorption dehumidification device comprises of dehumidifier, regenerator, vapor cooler, heat exchanger and pump. Liquid absorption dehumidification equipment boasts of a high processing capacity and a great dehumidification effect. Furthermore, the liquid drying agent can purify the air by absorbing such hazardous substances as partial pathogenic bacteria and chemical pollutants in the air in addition to the absorption of vapor. Liquid absorption requires the heat regenerated by the drying agent. Such heat can be obtained from low-grade heat sources, such as solar energy and industrial waste heat, which makes low energy consumption possible. But in this case, it is necessary to consider the stability of the heat source. The investment amount and coverage area will also change accordingly. As the overall coverage area of liquid absorption dehumidification equipment is greater than that of freezing dehumidification devices, constant maintenance is required. In view of relatively low COP of the system the corrosion of the drying agent to equipment and the control of liquid flow (prevention of droplets), this mode is mainly suitable for industrial application.
Just like liquid absorption dehumidification, solid absorption dehumidification is to use a drying agent to absorb vapor from the air. The only difference is that the drying agent is solid. The drying agent may release large quantities of heat during absorption of vapor. To maintain a great absorption capacity, it is necessary to cool the drying agent during absorption, which inevitably results in increased energy consumption. Reel absorption dehumidifier is the most typical solid absorption equipment, mainly comprising of drying reel, regenerating heater, dehumidification draught fan and regenerating fan. In the reel absorption dehumidification equipment, the damp air and regenerated air are delivered via the fans. And the rotation of the reel results in great noise. So regular mechanical maintenance is required. The higher the absorption capacity of the drying agent on the reel is, the higher the energy consumption during regeneration will be, and the higher temperature the regeneration process will require. Additional cooling equipment is required according to environmental requirements when necessary. Compared with cooling dehumidification, the solid absorption dehumidification technique features low COP and high dehumidification capacity, which is particularly applicable to treatment of air at low temperature and low humidity. The main applications are in industrial production processes.
With higher levels of production and increased living standards, awareness of environmental protection and energy conservation has been further enhanced. Various technical approaches and methods have been adopted to improve technological efficiency. And, more and more technological processes have been developed for greener or cleaner production. It is the same for dehumidification techniques. The development of green and environment-friendly processes and the technological innovation for energy conservation have become an inevitable development trend in the industry. In recent years, energy consumption and pollution (emission of CO2 and leakage of Freon), related to conventional air conditioning systems, have witnessed a continuous increase accompanied by an increase in the demand for air conditioning worldwide. Presently, the proportion of energy consumption for air conditioning is over 15% and increasing. Vapor content in the air varies significantly with regional features and the change of seasons. Due to the high latent heat produced by evaporation, dehumidification has become one of the main energy consumption parts of the air conditioning system, accounting for 20-40% of the total energy consumption of the air conditioning system. Improvements in dehumidification methods are an important step for energy conservation of air conditioning systems. Thermoelectric and cooling dehumidification techniques are based on the Peltire effect and the Beck effect, which is the application of thermoelectric refrigeration principles in the dehumidification process. This is characterized by small volume, high stability, no need of refrigerant or drying agent and environment-friendliness. This is an advanced technique in the field of dehumidification. Owing to the integration of the cooling and heating functions, thermal condensing dehumidification performs cooling and heating treatments simultaneously, resulting in low energy consumption. Compared with conventional condensing dehumidification systems, the overall power and energy consumption are significantly reduced. This system can use solar energy as the source of electric energy. And the adjustment of the condensing and heating effect through controlling the current flow is simply controlled. This ease of control ensures high stability for the whole dehumidification process. The thermoelectric device has a working life of over 100,000 hours, which far exceeds that of conventional condensing dehumidification equipment. With the exception of the low-power fan used for air flow and condensation, the equipment is free of mechanical transmissions throughout the whole dehumidification process; it features low noise, quick start-up and less stringent environmental requirements. As the ambient operating temperature is between −40 and 70° C., it is available for operation in extreme environments, and can be adapted to the designated working zones at will. In conclusion, this will inevitably become an important direction and method for the future development of dehumidification techniques.
However, the thermoelectric condensation dehumidification technology has the limits in that the boundary layer blocks the cold and heat transfer, and the mainstream flow cannot be efficiently heated or cooled, thus limiting the heat transfer and dehumidification capacity. For the traditional process, as shown in FIG. 1, according to hydromechanics, in the flow channels without flow control, both the upper and lower vane 110 would form the flow area. The air 170 flows in from the inlet at the left side. Starting from the inlet end, due to the adhesive property of the air, upper and lower boundary layers 160 form on the surface of the vane. The boundary layer shown in FIG. 1 represents the flow boundary layer. The fluid adjacent to the vane wall is at the same speed as the vane due to adhesion and generally static. Due to the existence of the flow boundary layer, a temperature boundary layer occurs, which is thicker than the speed boundary layer shown in FIG. 1. Due to the existence of the two kinds of boundary layer, the heat transfer between side wall and fluid is directly restrained. In addition, as the flow speed is progressively slower as it gets closer to the wall, this creates an environment for unclean fluid deposition on the surface of the vane. This impedes the heat transfer. Except the boundary area, the flow in the middle is the mainstream, which decides the characteristics of the whole fluid. Without effective control, the mainstream is of rapid and short flow based on the principle that fluid flows towards areas of the least resistance. This impedes the heat exchange of the integrated fluid with that clinging at the boundary and the overall heat transfer efficiency is reduced. Currently there is technical information regarding boundary layer flow control. See FIG. 2, the two structures are directly extruded from the wall plane. The shape of such structure can be adjusted randomly, but the size is small. The flow behind these structures is intended to disturb the boundary layer. The structure of these bulges can be varied, such as plane wing, cylinder, or cone, etc., but they are always very small. Therefore, for each single bulge, the flow or vortex created is minimal, and the flow disturbance very limited. In order to reach the goal of flow disturbance, many such bulges are needed but still does not effectively transfer the energy at the boundary into mainstream, failing to correct the problem of mainstream short flow. With a traditional cooling and dehumidification method, after the moisture is eliminated, the gas temperature might drop too much to satisfy users' requirements on temperature. And the traditional way also results in energy waste for reheating of the gas, generally equaling to cool the consumed energy. This doubles the waste of energy. This is irrational and in urgent need of solution.